Integrated circuits are manufactured as assemblies of various devices, such as transistors that make up a chip. In the process of manufacturing integrated circuits, after the individual devices, such as transistors, have been fabricated in the silicon substrate, the devices must be connected together to perform the desired circuit functions. This connection process is generally referred to as “metallization”, and is performed using a number of different photolithographic and deposition techniques.
Contact plugs are employed to make a solid connection between an underlying device, for example, and an overlying interconnection conductive line, for example. The fabrication of a contact typically involves forming an opening in the dielectric layer and filling the opening with a metallic layer, such as aluminum or tungsten. However, aluminum or tungsten ions from the contact can migrate into a silicon substrate through a doped region, causing a short to the substrate. To minimize this shorting, many processing techniques deposit a barrier layer before depositing the aluminum or tungsten. One type of common barrier material is titanium nitride (TiN). While titanium nitride has a good barrier ability, it needs to be thick enough to effectively function as a barrier layer. As integrated circuit devices are defined more finely, the diameter of the contact shrinks and becomes more critical. Thus, a thick titanium nitride barrier metal layer is less desirable in more highly integrated circuits.
Another commonly used barrier layer is formed from metal organic CVD titanium nitride (MOCVD-TiN). Inherently, the MOCVD-TiN material contains impurities such as carbon, hydrogen and oxides, so that the resistance of MOCVD-TiN material is high. In order to reduce the resistance, one method removes these impurities by treating the barrier layer with a plasma gas containing an atmosphere of nitrogen and hydrogen. However, following the plasma gas treatment, the thickness of the MOCVD-TiN is substantially reduced. Consequently, the treated MOCVD-TiN layer thus formed has comparatively lower resistance, but the thickness should be adequate to function as a barrier layer effectively.
The effectiveness of the contact is limited by the contact resistance between the barrier metal layer and the doped regions in the substrate. Contact resistance is of particular concern in CMOS (complementary metal-oxide-silicon) technology. One approach to reduce the contact resistance is to deposit a conformal refractory metal layer into the opening, deposit the barrier metal on the refractory metal layer, and then perform a thermal process to effect a reaction between the metal and the silicon atoms of the substrate to form metal silicide. Since the metal silicide has a low resistance, consequently the contact resistance can be reduced.
One of the concerns of this methodology is that the plasma treatment to densify or reduce contaminants in the MOCVD-TiN layer affects the contact metal (titanium) underneath, especially in the case when titanium silicide formation is required for low contact resistance to the silicon substrate. This is due to the top portion of the titanium being easily converted to titanium nitride prior to being converted to titanium silicide in a subsequent rapid thermal anneal (RTA) step. The conversion of portions of the titanium to titanium nitride causes degradation of the contact resistance and distribution. Furthermore, in order to provide adequate titanium to form silicide, and compensate for the conversion of a portion of the titanium to titanium nitride, an excessive amount of titanium needs to be initially deposited. However, this raises concerns regarding overhang on the contact opening, as well as requiring additional titanium material for the process.
FIG. 2 depicts an N2/H2 plasma treatment in which the contact hole 20 has been provided with an initial titanium contact layer 22 followed by a barrier metal titanium nitride layer 24. This is provided on a silicon substrate 26. The N2/H2 plasma treatment is performed to reduce the resistance and remove impurities in the titanium nitride layer 24. However, as seen in FIG. 3, a portion of the titanium layer 22 is converted by the N2/H2 plasma treatment to an additional titanium nitride region 28. This reduces the amount of titanium that can react with the silicon to form silicide. This results in the structure of FIG. 3 following an annealing step to form silicide 30. The penetration of the N2/H2 plasma into the titanium through the titanium nitride layer 24, as shown in FIG. 2, therefore reduces the thickness of the titanium silicide 30 that is ultimately formed. Thus, in order to provide an adequate titanium silicide region, an excessive amount of titanium may be deposited to compensate for the conversion of some of the titanium to titanium nitride by the plasma treatment.